Allusions to the level up, FP, and BP systems. (Minor, I guess... This would be in the manual.)
Spin dashing instructions. (Also minor, I suppose. Although there will be a more natural time to bring this up - when Speedy Spin appears or he starts playing around with it while moving.)
When you unlock action commands.
There's an "expansive area" in chapter 2.
Telling him to go back to the tree with the doll.
The power of Power Bounce. People really can't take off their experienced-player-lenses, huh?
The difficulty of certain bosses (chapter 6).
Anti Guy warnings.
Level grinding in Flower Fields. Now with allusions to the golden flower!
Goombella
from TTYD.
Curses
in TTYD.
Peekaboo and Quick Change.
That you can visit Luigi at Mario's house.
Minh T.'s quest isn't optional.
The existence of multi-hit attacks and how Zap Tap/Volt Shroom interacts with them. Now people have brought up the Fuzzy interaction too.
Luigi's floorboards have been pointed out. And now someone spelled it out, before we even have the ground pound.
The experience system was explained. I guess this is minor, but it's also something he'd discover himself if people just let him.
Trick to the spinning pole secret.
Allusions to cooking and some items being more useful as ingredients than for direct use. Now with hints about when it is unlocked!
The number of HP and FP Plus badges in the game.
Bombette's gender (but not name, at least) one episode before he'll get her because ???
Sushie-Bob-omb interaction. While worded such that it reveals basically nothing about Sushie, which is good, it does hint at the fact that there are no advanced Bob-ombs later in the game.
The existence of hidden blocks. Like ... I guess this isn't a spoiler, but the game will, if nothing else, tell him when he gets Watt...
Partner damage mechanics. A guide thing, I suppose, but again, earlier than necessary.
Kooper will have something to say in chapter 7.
There are
Bob-omb and Koopa
partners in TTYD.
Hints at eventual fast travel. Now fully described, just before he'd likely find it.
Discussion of how partners perform when upgraded even though the game has not introduced such upgrades yet.
The effects of Dodge Master. Now including name!
Will eventually get a ground pound.
There's a
train section
in TTYD.
The fighting style of the "next partner" (Parakarry, in this instance).
Ultra Shrooms and Jammin' Jellies.
The Cookbook and its effects. Including the more obscure one of changing some single-item recipes.
Implied cost of Defend Plus. Which also implies existence, of course.
There's a
Yoshi partner
in TTYD.
Hints that he found the ruins room in the desert before getting the necessary key item.
Tweesters are used to get on top of otherwise unreachable platforms.
Buzzar was a boss fight if he hadn't answered "Luigi".
All of Merlee's effects, even though he had only seen one of them so far.
Hidden room in Dry Dry Ruins.
Sure, just blurt out the names of later bosses (General Guy and Crystal King) in chapter 2 comments lol
Allusions to Peach's point of no return. Thankfully very vague.
Jump upgrade is coming up. Kind of obvious, but...
Goombario's charge has a more practical use later (that is, Multibonk).
There's an important secret in Forever Forest for progression later in the game.
We're about to meet a fan favorite character (Bow)!
I Spy allusions. ...this one might be for the best, though.
There's a third tier of hammer.
And why not the second tier of partner upgrades, while we're at it!
We'll get the partner "Spike" later.
Goombario gets Multibonk (remember that we don't know about ultra rank yet).
Master 3's stats are on the level of the final boss.
There are more optional Anti Guys later.
No Shy Guy partner. Mild, but could have waited until getting Watt, no?
Various two-item recipes.
Rough distribution of fire enemies for the rest of the game. That is, chapter 5 onward.
Last Stand.
We'll eventually face enemies obviously weak to fire.
All variants (including future ones) of Magikoopas have 11 HP.
Kent's tail has no defense and he's weak to sleep.
Won't be able to increase BP again now that he's hit 30.
Rip Cheato sells multiple items, but only the first 10 are worthwhile.
There are Ninjis in this game. Thanks shoutbox.
Triple Dip is somewhere in the Crystal Palace.
Final boss also heals.
Anti Guy Unit exists. At least they didn't say what triggers the battle.
Ice (Start of Chapter 7)
#1 mega cd enjoyer AND "making fun of"-er 
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